Electrolytic processes for the production of thin ferromagnetic film



Jan. 13, 1970 p, c z ETAL 3,489,661

ELECTROLYTIC PROCESSES FOR THE PRODUCTION OF THIN FERROMAGNETIC FILM Filed March 24, 1966 United States Patent M Int. or. 622m 5/32 US. Cl. 204-43 1 Claim ABSTRACT OF THE DISCLOSURE Process for preparing, by electrolysis, a homogeneous thin film of a ferromagnetic alloy deposited on a metallic metallized support used as a cathode, at a constant current density, at a constant temperature and without stirring, and using an aqueous electrolyte solution including ions of the constituent metals of the deposited alloy, the concentration ratio of the metal ions in the electrolyte being substantially equal to the concentration ratio of the metal atoms in the alloy to be obtained having zero magnetostriction, said electrolyte further containing a given thiourea content. The electrolyte has a pH of about 2.5 and the valve of the current density is chosen in relation to said temperature, so that the deposited alloy is of constant composition and exihibits a magnetostriction of substantially zero value.

This invention relates to thin ferromagnetic films, also called thin ferromagnetic foils or skins, and electrolytic processes for the production of such films. The invention concerns more particularly, but not exclusively, the production of films of this type which are intended to form memory elements having very rapid triggering times in high-capacity magnetic memories.

The invention has for one object to adapt such electrolytic processes so that they conform better than hitherto to the various practical requirements, notably in regard to the production of very thin ferromagnetic films or foils which are homogeneous throughout their thickness and in the plane of the film, and which have a weak anisotropy field and a weak coercive field, notably of minimum value, and reduced changes in the direction of easy magnetisation and the strength of the anisotropy field.

The essential particulars regarding ferromagnetic substances, notably in the form of thin films, and more particularly the notions of direction of easy magnetisation and anisotropy field, which are necessary for an understanding of the invention are set out in the patent application of Pierre Georges Henri Chezel et al., Ser. No. 536,998, filed Mar. 24, 1966, hereinafter called the first application." It will be recalled on the other hand that the coercive field of a ferromagnetic substance having a hysteresis loop is the absolute value of the magnetic field which is necessary for cancelling out the magnetisation. The macroscopic coercive field is here concerned. There also exists a microscopic coercive field, which is the field which must be overcome in order to obtain the reversal of the magnetisation in the domains of the ferromagnetic substance (the domains are also being defined in the said first application).

In accordance with a principal feature, the invention comprises producing the deposition of a thin film or foil of a ferromagnetic alloy by electrolysis with constant current density at constant temperature and Without agitation, by means of an electrolyte having a high concentration of ions of the constituent metals of the alloy to be deposited, the ratio of the concentrations of the metallic 3,489,661 Patented Jan. 13, 1970 ions of the electrolyte being substantially identical to the ratio of the concentrations of the metallic atoms of the said alloy, and containing a particular quantity of an additive such as thiourea, which displaces the polarisation curves of the deposit of the constituents of the alloy on the said alloy in order that these curves may intersect one another at this current density and at this temperature.

Apart from this principal feature, the invention comprises other features which will preferably be used at the same time, but which could, where necessary, be used singly and which will hereinafter be more explicitly referred to.

The invention concerns more particularly a certain mode of application (that in which it is applied to the preparation of one of the thin layers of a pair of coupled ferromagnetic layers of the type described in the copending patent application of Pierre Georges Henri Chezel et al., Ser. No. 536,997, filed Mar. 24, 1966, hereinafter referred to as the second application), as also certain embodiments of the said features, and it concerns more particularly still, as new industrial products, the thin ferromagnetic films or foils obtained by the aforesaid process, as also the electrolytic installations in which this process is carried out and the devices (magnetic memories and logic or parametric devices) comprising such films.

For a better understanding of the invention, and the manner in which it may be carried into effect, the same will now be further described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 illustrates the variation of the iron content of a nickel-iron alloy deposited, as a function of the current density, when an electrolyte is employed which contains nickel and iron sulphates and thiourea, and

FIGURE 2 illustrates the displacement of the polarisation curves produced by the addition of thiourea in the case of the deposition of iron on iron, on the one hand, and of nickel on nickel on the other hand.

In carrying out the invention, and more especially in accordance with that mode of application and those em bodiments of its various parts which appear to be preferable, the following procedure or a similar procedure is adopted, for example, for preparing thin ferromagnetic foils, layers or films.

However, before examples of the process according to the invention is described in detail, the properties generally required of a ferromagnetic film employed as a memory element and the difficulties generally encountered in depositing such films by electrolysis will be briefly indicated.

The ferromagnetic properties of certain metal alloys, more particularly alloys of nickel and iron, where necessary with cobalt, vary in accordance with the proportions of the constitutents. More particularly, the magnetostriction and the anisotropy field vary as a function of the composition of the nickel-iron alloys, as set forth in detail in the said first application.

Now, in many applications, notably for the production of magnetic memory elements, it is desirable to prepare films whose magnetic characteristics depend only very slightly upon the stresses exerted on the film, i.e. films having zero or low coefficients of magnetostriction. It will be recalled that these alloys, of the Permalloy type containing 8283% of nickel and 18-17% of iron, have zero magnetostriction and at the same time a minimum anisotropy field.

The reduction of the anistropy field is also desirable because this anistropy field is related to the microscopic coercive field and the currents controlling the triggering of a memory element increase with the anisotropy field.

Finally, it will be recalled that the magnetisation is generally oriented in a privileged direction of easy magnetisation by effecting the deposition of the film in the presence of a magnetic field of constant direction, so as to induce a uniaxial anisotropy. In this way, a layer is substantially obtained which comprises only one direction of magnetisation, ie only one domain. The real magnetisations are slightly dispersed in direction in relation to the mean direction of the magnetisation, while the anisotropy fields are slightly dispersed in intensity.

In order to permit correct recording of the information and above all preservation of the information after repeated interrogations or disturbances, it has been found that it is advantageous to produce films having a very small thickness, i.e. below 300 A., because Bloch lines (along which the orientation of the magnetisation changes within a Neel wall or cross-tie) commence to appear when the thickness exceeds 300 A. Now, the propagation or creep of the Bloch lines is likely to destroy the stored information.

These thin films must in addition have a very good homogeneity, so that the sum of the microscopic and macroscopic angular dispersions of the magnetisation is less than two or three degrees, and the microscopic dispersion of the intensity of the anisotropy field is less than two or three percent. It is even highly desirable to prevent the formation, in a thin film, of zones (which are never numerous, but are troublesome when they exist) in which the direction and magnitude of the anisotropy field depart greatly from the mean direction, because these zones may generate dynamic blocking fields during the interrogation by means of high-frequency pulses (the existence of such zones makes it necessary to apply a magnetic field which is much higher than that calculated from the mean anisotropy for effecting a truly total rotation of the magnetisation by a brief field pulse, of the order of several tenths of a nanosecond, in the direction of difficult magnetisation) Hitherto, a number of methods have been proposed for preparing ferromagnetic films, notably evaporation in vacuo, cathode sputtering, and ion plating, which was re cently developed by the American Atomic Energy Commission in the Sandia Laboratory, chemical decomposition in the gas phase, and finally electrolysis.

Electrolysis has the advantage that its cost is low, but on the other hand it has a number of disadvantages in the previously known methods of application, including notably:

The current density on the foils or supports constituting the cathode varies in the course of the electrolysis owing to poor conductivity of the support, so that it is necessary to choose the operating conditions in such manner that the composition of the deposit depends as little as possible upon the current density and upon the thickness of the deposit;

Some evolution of hydrogen occurs on the electrodes, simultaneously with the deposition of iron and nickel; now, this gas is detrimental to the magnetic properties of the deposited alloy; generally, it is arranged to use a high current density, because the quantity of hydrogen evolved decreases when this current density increases;

Particularly in the case of the deposition of an ironnickel alloy, the initial deposit has a much higher iron content than the electrolyte, which results in an unequal relative depletion of the nickel and iron ions in the electrolyte in the neighborhood of the electrode, whereby a composition gradient through the thickness is produced; the result of this is that the first layers have not zero magnetostriction, which results, on the one hand, in sensitivity to external stresses and deformations, and on the other hand in the impossibility of obtaining a well-defined direction of the axis of easy magnetisation. This gradient also has the effect of raising the mean anisotropy energy, because the anisotropy energy is minimum with zero magnetostriction level, as indicated in the first application. An attempt is made to obtain in practice a deposit whose concentration is as regular as possible throughout the .4 thickness (so as to produce a magnetostriction which is constantly as low as possible and a mean anisotropy which is as close as possible to the minimum) by maintaining an agitation of the electrolyte in order to compensate for the relative reductions of concentration of the latter, but unfortunately such agitation destroys the homogeneity of the magnetic properties in the plane of the deposit;

Fluctuations in composition occur in the plane of the foil, owing to the fluctuations of the activities of the support and the current peak phenomena; especially when they are associated with stresses in the deposit, these fluctuations have the effect of locally deflecting the magnetisation by magnetostriction;

It is ditlicult to obtain crystals of uniform size and to distribute these crystals in an isotropic manner, the presence of large crystals and a non-isotropic distribution having the efiect of locally deflecting the magnetisation owing to the anisotropy energy or magnetocrystalline energy.

In fact, the essential disadvantage of the usual electrolysis arises out of the fact that the relative metal contents are different in the electrolyte and in the alloy to be deposited. This results in the formation of an involuntary or uncontrolled gradient in the composition of the alloy deposited in the direction of the thickness and consequently in the production of a deposit which has not minimum anisotropy.

The present invention has for its object to obviate the aforesaid disadvantages by effecting the deposition of a film or thin foil of a ferromagnetic alloy, notably of a nickel-iron alloy and more particularly an alloy containing 82-83% of nickel and 18-17% of iron with zero magnetostriction, by electrolysis at constant current density, at constant temperature and without agitation by means of an elecrolyte which has a high concentration of ions of the constituent metals of the deposited alloy, the ratio of the metallic ion concentrations of the electrolyte being substantially identical to the ratio of the metallic atom concentrations of the said alloy, and containing a certain quanity of an additive such as thiourea, which displaces the polarisation curves of the deposit of the constituents of the alloy on the said alloy so that these curves intersect one another at this current density and at this temperature.

Preferably, the electrolyte contains the metallic ions in the form of sulphates, the total concentration of sulphates, notably in the case of the deposition of a nickeliron alloy having zero magnetostriction, being several hundred grammes of sulphates per litre of electrolyte.

Example As a non-limiting example'of the application of the invention, the following electrolyte was prepared, in which the ratio of the concentrations of ferrous and nickelous ions is identical to that of the aforesaid alloy containing 82-83% of nickel and l8l7% of iron:

Nickel sulphate NiSO .7H O 462.5

Iron sulphate FeSO .7H- O 98.7 Boric acid H BO (buffer) 30 Sodium lauryl sulphate (wetting agent) 0.420 Thiourea (regulator) 0.250 pH 2.5

The quantities indicated are in grammes per litre of electrolyte.

The electrolysis is carried out on a metallic or metallised support serving as cathode, with constant current density (6.8 Ina/cm?) at constant temperature (28 C. for the indicated quantity of thiourea) and without agitation, the electrolyte being advantageously maintained under an inert atmosphere (nitrogen atmosphere) to avoid oxidation of the iron in the air. The anodic oxidation of the iron is suppressed by the use of an anodic compartment separated by a diaphragm from the cathodic compartment and containing an electrolyte free from iron and nickel (sodium sulphate and boric acid at pH=2.5).

The high molarity in nickel and iron salts limits the harmful effects of the diffusion, through the electrolyte, in the domain of the current densities employed.

Thiourea, in addition to having the essential advantage of permitting the production of a deposit having the same metallic composition as the electrolytic bath (for the reasons explained in the following), increases the yield of the electrolysis, which may reach and even exceed 95%, with a resultant considerable lessening of the dangers of pollution of the deposit by hydrogen (this being due to the phenomena of displacement of the polarisation curves of iron, nickel and hydrogen which are discussed in the following with reference to FIGURE 2).

One of the essential advantages is that a composition is obtained which is the same throughout the thickness of the layer, substantially without any composition gradient in the direction of the thickness.

The composition of the deposit is essentially a function of the current density, the temperature and the thiourea content of the electrolyte. More particularly, there can be obtained from the aforesaid electrolyte, by operating without agitation, at a given temperature and with a given thiourea content, a deposit whose composition varies as a function of the current density from pure nickel to an iron content at least equal to that of the electrolyte.

In FIGURE 1, the curve 1 illustrates the variation of the iron content (plotted along the ordinates in percent) of the deposited nickel-iron alloy as a function of the current density i (in ma./cm. plotted along the abscissae) for the aforesaid electrolyte at 28 C. The experimental points noted are represented by crosses 2.

It will be seen from FIGURE 1 that, for a current density of the order of 6.8 ma./cm. (i.e. that mentioned above), the alloy containing 82-83% of nickel and 18- 17% of iron with zero magnetostriction and minimum anisotropy field is obtained, from an equal concentration ratio of ferrous ions and nickelous ions in the electrolyte, that is to say, without relative depletion of one ion or the other in the electrolyte. Analyses have shown that the aforesaid composition of the alloy was obtained with the first fifty angstroms, i.e. in the course of the period of deposition in which the influence of the preparation of the surface of the support is essential, notably as a result of the phenomenon of epitaxy. It is also known that the final characteristics of the complete deposit depend to a large extent upon those of the initial layers.

When working under these conditions, i.e. without agitation, at 28 C. and with 6.8 ma./cm. and 0.250 g. of thiourea per litre of electrolyte, there is finally obtained a deposit whose composition substantially does not vary in the direction of its thickness.

There are also represented in FIGURE 1 by crosses 3 surrounded by a circle the operating points at which the nickeliron alloy is deposited with zero magnetrostriction, i.e. without relative depletion of the ferrous or nickelous ions in the electrolyte, each of these crosses corresponding to a temperature (in C.) and current density (in ma./cm. couple. It will be seen that the higher the temperature of the bath, the higher the current density must be in order to obtain the alloy with zero magnetostriction.

In FIGURE 2, there is shown the displacement of the nickel-on-nickel and iron-on-iron polarisation curves under the influence of the addition of thiourea. In FIGURE 2, the polarisatio Ec in millivolts in relation to the reference electrode has been plotted along the abscissae, and the current densities i in ma./cm. has been plotted along the ordinates. The curves 4 and 5 represent the nickel-on-nickel polarisation for an electrolyte without thiourea and for the electrolyte containing 0.500 g. of thiourea per litre, respectively. Likewise, the curves 6 and 7 represent the iron-on-iron polarisation for an electrolyte without thiourea and with 0.500 g. per litre of thiourea, respectively, the points 8 representing the experimental results. There will be seen a displacement towards the left, i.e. towards the positive potentials, under the influence of the thiourea (that is, when passing from a chin-lined curve without thiourea to a solid-lined curve with thiourea). It will also be seen that the curve of the deposition of iron-on-iron is much less displaced towards the positive potentials (displacement of the curve 6 towards the curve 7) than that of nickel-on-nickel (displacement of the curve 4 towards the curve 5), which probably explains the readjustment made in the ratio of the concentrations of the deposited alloy.

In fact, the essential action of the thiourea is to displace towards one another the curves representing the deposition of iron on a nickel-iron alloy having zero magnetostriction, and of nickel on the same alloy in order that these two curves may intersect one another under certain operating conditions without agitation, i.e. for the temperature-current density couple indicated at 3 (crosses surrounded by a circle) in FIGURE 1.

It has also been found that the quantity of thiourea present in the electrolyte has an influence on the internal stresses in the deposited alloy. Depending upon its concentration, thiourea may produce variable stresses and even stresses of different signs. Consequently, there is an optimum quantity of thiourea to be added to the electrolyte, which depends upon the difference between the crystal meshes of the base metal co nstituting the support and of the deposited alloy, as also the thickness of the deposit. It is consequently useful, when it is desired to deposit alloys other than that specifically referred to by way of example, which contains 82-83% of nickel and 18-17% of iron, to determine beforehand the quantity of thiourea to be added to the electrolyte in order that the contribution of the magnetostriction to the dispersion may be very low.

The production of uniformity in the size of the crystals also has the effect of eliminating the contribution of the magnetostriction to the dispersion and it has been found that the presence of thiourea adsorbed on the thin ferromagnetic layer or foil in the course of the preparation produces a perpetual reorientation of the crystal nuclei, which results in a formation of very small crystals which are isotropically distributed, whereby the current peaks are eliminated.

With the electrolyte of the aforesaid example at 28 C. under 6.8 ma./cm. the desired composition of 82-83% of nickel and 18-17% of iron was obtained, with an electrolysis yield of more than in the first fifty A. with the following magnetic properties for a layer thickness between and 250 A.:

Anisotropy field oersteds 2.5 Coercive field do 2.5 Microscopic angular dispersion 2 Macroscopic angular dispersion 1 It was found that sulphur was present in this film (the sulphur emanating from the thiourea).

There will be seen in FIGURE 1 a level of the iron concentration of the deposit as a function of the current density: the deposit substantially retains the same composition if the current density varies slightly. Applicants investigations have shown that this level, which corresponds to an iron content of the order of 65% for an electrolysis temperature of 28 C. with the electrolyte of the example, corresponds to an iron content of the order of 18-17% (that of the alloy having zero magnetostriction) for a temperature of the order of 60 C. with the same electrolyte.

It may be advantageous to work under these conditions in order to avoid the variations of composition due to the variations of current density.

The characteristics of the thin films obtained under the level conditions and under the conditions of the example are the same: zero magnetostriction, minimum anisotropy, homogeneity, etc.; in addition, the magnetic fields necessary for pulse-wise triggering are identical to the necessary static fields.

Thus, regardless of the embodiment adopted, there is always obtained a process for the production of thin ferromagnetic films, the application of which is sufliciently clear from the foregoing to require no further explanation, and which has, as compared with the earlier methods of electrolysis employing an electrolyte of difierent metallic composition from the deposit, many advantages including notably the following:

The composition of the deposit constituting the thin film substantially does not vary in the direction of the thickness owing to the fact that the electrolyte has the same ferrous and nickelous iron composition as the deposit to be produced;

The deposit is well-oriented and the mean anisotropy energy is low;

The magnetostriction is substantially zero;

It is possible notably to deposit very thin layers (of a thickness of less than 300 A.) which are homogenous and have zero magnetostriction and minimum anisotropy;

It is possible to choose, if desired, an electrolysis temperature such that the variations of current density result only in an insignificant variation of the composition of the deposit, so that it is possible to eliminate fluctuations of concentration in the plane of the deposit, which fluctuations would tend to increase the dispersion of the direction of easy magnetisation of the deposit and to produce the blocking of the coherent rotation of the magnetisation in pulse operation; in the case of a deposit of a ferromagnetic film on a wire, the variation of the crosssection of the wire, which produces a variation of the current density, does not result in any variation of the composition;

Stresses in the deposit are eliminated by the presence of a well-defined quantity of thiourea; consequently, the zones whose local composition might be different from the composition corresponding to a zero magnetostriction, are freed from any stress and do not produce any dispersion in relation to the desired direction of easy magnetisation and blocking of the coherent rotation in pulse operation;

Thin magnetic films are obtained in which the ratio of the coercive field to the anisotropy field is at least equal to 1.

It will be self-understood, and will also be apparent from the foregoing, that the invention is in no way limited to those modes of application or to those embodiments of its various parts which have been more particularly considered, but covers all variants thereof.

More particularly, it would be possible without departing from the scope of the invention to deposit by the process of the invention not only nickel-iron alloys other than that containing 18-17% of iron, but also alloys other than nickel-iron alloys, notably binary and ternary alloys of Fe, Ni, Co and M0, by replacing the nickel and iron ions in the sulphates of the example by those of the desired alloy and modifying the quantity of additive (thiourea). On the other hand, the process of the invention is applicable to the deposition of alloys both upon plane supports and upon filiform supports.

We claim:

1. A process for preparing, by electrolysis, a homogeneous thin film of a ferromagnetic alloy deposited on a metallic or metallized support used as a cathode, at a constant current density, at a constant temperature and without stirring, which comprises using an aqueous electrolyte having the following composition per litre:

(a) NiSo '7I-I O about 462.5 grams (b) FeSO -7H O about 98.7 grams (0) boric acid about 30 grams (d) sodium lauryl sulphate about 0.420 gram (e) thiourea about 0.25 gram said process further comprising operating at a pH of about 2.5, at a temperature of about 28 C. and at a current density of about 6.8 ma./cm.

References Cited UNITED STATES PATENTS 3,032,486 5/1962 Sallo et al. 204--43 3,047,475 6/ 1962 Hespenheide 204-43 3,098,803 7/1963 Godycki et al. 20443 XR 3,124,520 3/ 1964 Juda 204252 XR 3,239,437 3/1966 Stephen 20443 3,261,711 7/1966 Sallo 106--1 XR OTHER REFERENCES IBM Technical Disclosure Bulletin, p. 53, vol. 3, No. 2, July 1960.

Brenner, Abner, Electrodeposition of Alloys, pp. 239, 242, 243, 267-271, 274, and 275, vol. II, 1963.

Graham, A. Kenneth, Electroplating Engineering Handbook, pp. 524-525, 2nd edition, 1962.

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner 

